This invention relates to an RF amplifier intended for operation over a wide band, for example, 1.6-18Mhz and it is an object of the invention to provide an improved RF amplifier of this nature.
In the design of quality communication receivers, for the high frequency bands, the prior art has usually relied on large cumbersome multi-wafered band switches to accomplish the necessary switching of tuned circuits for each segment of the receiver's operating range. Such receivers were, accordingly, large, cumbersome and expensive. Even so, the receivers had to deal with the ever present problems of intermodulation, cross modulation and blocking, particularly in the instances where a relatively small desired signal was in the particular band within which a relatively large undesired signal existed.
Simplifying communication's receivers of the nature indicated required that the initial or input RF amplifier stages have a wide band response, such as from 1.6 to 18Mhz, and have that response essentially linear throughout that range. Transformers receiving all frequencies over the indicated band have to provide an output impedance of relatively high value and have a linear response over this same frequency range. The transformers also need to be capable of handling large signal levels such, for example, as an RF input of one volt RMS with no degradation of performance.
Satisfactory operation of an RF communications receiver in an environment of high level RF signals requires that protection of some nature be present near the receiver input to protect against the generation of intermodulation distortion products, cross modulation distortion products and de-sensitization or blocking in the first active stage or stages of the receiver, such as the first amplifier. Besides the use of band pass, low pass, or high pass filters at the immediate input to the receiver, a variable RF attenuator located between such filters and the first amplifier would limit the remaining off channel high level signals to tolerable levels. According to the invention, an attenuator of the shunt-series-shunt configuration utilizing J-Fet semiconductor devices can provide a range of attenuation from 45db to 1db depending upon the value of the input signal. The amplifier proper for the required linearity may be of the MosFet type which operates most satisfactorily at relatively high impedance levels. Such a high impedance level is achieved by the J-Fet attenuator and by the transformer which supplies the signals to the input of the attenuator.